The present invention relates to a method of manufacturing a plastic windshield for use on a vehicle such as an automobile, and a packing for use in a mold assembly employed by the method.
Windshields for use on vehicles such as automobiles are generally manufactured by shaping glass sheets to desired curved configuration. Such glass sheets are produced by a floating process in which molten glass flows into a bath containing molten tin or the like. Since the glass sheets thus produced are highly uniform in thickness, any see-through defects that are present in the glass sheets are substantially negligible.
The automobile windshields of glass have proven unsatisfactory, however, with respect to weight, shock resistance, and other properties, although they allow high visibility and transparency.
Attempts have been made to manufacture windshields of plastic materials. Thermoplastic resins such as acrylic resins, polycarbonate resins, and the like which are well moldable are not suitable for use as windshield materials since they are less resistant to abrasion. Thermosetting resins such as diethylene grycol bisallyl carbonate have satisfactory hardness and transparency levels, but are less moldable.
More specifically, when a thermosetting plastic is to be molded, a monomer plastic material is poured into a mold, and then heated so as to be set due to polymerization. Since such a thermosetting plastic material shrinks when it is set, seethrough detects are liable to occur in the windshield produced.
For molding a sheet member such as a windshield of a thermosetting plastic material, it has been customary to sandwich a packing of polyvinyl chloride or ethylene-vinyl acetate between two glass sheets to define a mold cavity therebetween, and then to pour a thermosetting plastic monomer into the mold cavity. Thereafter, the thermosetting plastic monomer is heated so as to be set due to polymerization.
The packing used in the conventional molding process is generally of a solid circular cross section or a hollow circular cross section. It has been quite difficult to position the packing properly between the glass sheets, and hence it has been time-consuming to mold a product to accurate shape. If a large-size product such as an automobile windshield is molded by the mold, the material monomer may sometimes leak through gaps between the mold glass sheets and the packing.
The conventional packing fails to follow the shrinkage of the thermosetting plastic material as it is set under heat. The molded product therefore is irregular in thickness and suffers cracks and/or undulations on its surfaces.